Easy.

Unfold and smooth your foil- smooth foil heats more efficently.

TIGHTLY fold it in half for strength (optional, I like redundency), shiny side in.

Fold the sides up, about a thumbslength deep, to form the sides.
(If not doubled, shiny side IN, so you heat faster)

Pinch the corners flat.

Add water, put over fire. Watch it, becuase if you boil it dry, you might burn a hole in your foil if it is on the lighter side of things. Even if you don't, all that good water, gone up in steam.

Basically, try to imitate a loaf pan.